C++26 Contracts: Introduction to Preconditions and Postconditions

C++26 introduces a new contracts feature that allows developers to specify preconditions and postconditions directly in function declarations using `pre()` and `post()` syntax, making requirements visible in the API rather than hidden in implementation details. Unlike traditional `assert()` statements, which are controlled by the NDEBUG macro, contracts offer more flexible checking options including ignoring violations, reporting and continuing, or terminating the program. This approach improves code clarity by making function expectations explicit to anyone reading the API.